[Amps] Alpha 76A metering changes

[k7fm]

Like many amplifiers, the Alpha 76A (and variants), does not have a grid 
trip circuit.  Years ago I installed build a board that did a number of 
tasks in addition to a grid trip, including SWR trip, peak reading and a few 
other things.  However, it was plagued with rf so I ripped it out.  One of 
the difficulties with the normal simple grid trip circuit is that the grid 
metering is above ground (by something like 5.6 volts).

The real reason I wanted grid trip was because I might have the multimeter 
switched to rf power output and because both the rf output meter and the 
grid trip meter have the normal analog meter delay.

There is a fine grid trip circuit on the internet for the 76A, however I 
took a different path and it works fine.

FIrst, I installed a peak reading wattmeter circuit in the rf output 
circuit.  It runs on 12 volts, but I installed a small 12 volt regulator 
running off the 28 volt supply.

Secondly, I installed an led array between the two panel meters.  No holes 
need be drilled.  This array is driven by an LM 3914 led driver, which is 
configured as a peak reading meter, which monitors the grid current.  This 
is a simple circuit.  Because the circuit is above ground, it requires a 
separate supply.  That was easy.  I found a small wallwart and opened it up, 
then mounted it to an aluminum bracked attached to the back panel.  So, no 
matter what position the multimeter is switched to, I can monitor peak grid 
current visually. Both will insure that I am not overdriving the amp.

Both circuits work fine.

73,  Colin  K7FM